1. Field of the Invention
The present invention relates to a driving circuit for a light emitting diode.
2. Description of the Related Art
In recent years, as a backlight of a liquid crystal panel or as an illumination device, a light emitting apparatus is employed, which is configured using an LED (light emitting diode). FIG. 1 is a circuit diagram which shows a typical configuration of a light emitting apparatus. A light emitting apparatus 1003 includes multiple LED strings 1006_1 through 1006_n, a switching power supply 1004, and a current drive circuit 1008.
Each LED string 1006 includes multiple LEDs connected in series. The switching power supply 1004 boosts an input voltage Vin, and supplies a driving voltage Vout to one terminal of each of the LED strings 1006_1 through 1006_n.
The current drive circuit 1008 includes current sources CS1 through CSn which are respectively provided to the LED strings 1006_1 through 1006_n. Each current source CS supplies, to the corresponding LED string 1006, a driving current ILED that corresponds to the target luminance level.
The switching power supply 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 controls the on/off duty ratio of the switching transistor M1 so as to adjust the driving voltage Vout.
With such a light emitting apparatus 1003, in some cases, in order to adjust the luminance level of each LED string 1006, a PWM (Pulse Width Modulation) control operation is performed on the on period TON and the off period TOFF of the driving current ILED. Such a control operation is also referred to as the “burst dimming control operation” or “burst control operation”. Specifically, a burst controller 1009 of the current drive circuit 1008 receives pulse signals PWM1 through PWMn each having a duty ratio that corresponds to the luminance level so as to perform a switching control operation on the respective current sources CS1 through Csn.
[Related Art Documents]
[Patent Documents]
[Patent document 1]
Japanese Patent Application Laid Open No. 2010-015967
[Patent document 2]
Japanese Patent Application Laid Open No. 2009-188135
If the driving currents ILED1 through ILEDn of the respective channels have uniform phases in the burst dimming operation, the output current Iout of the switching power supply 1004 concentrates at particular timings. In some cases, this becomes a factor contributing to ripple in the output voltage Vout or a cause of undesired noise. This problem can be solved by an arrangement configured to input the burst control signals PWM1 through PWMn having phases shifted from one another such that the on periods TON of the respective channels each have a different time offset.
However, with such a method (which is referred to as the “phase shift burst dimming method”), there is a need to generate the burst control signals PWM1 through PWMn by means of a processor (DSP) external to the light emitting apparatus 1003, which imposes a heavy burden on the designer of liquid crystal TVs.
Furthermore, in a case in which there is a desire to make a design change with respect to the number of channels of LED strings, there is a need to change the design of the circuit configured to generate the burst control signals PWM1 through PWMn This leads to a problem of increased design costs.